Update v8
Below answers work but exposes your application to XSS security risks!.
Instead of using this.domSanitizer.bypassSecurityTrustResourceUrl(url), it is recommended to use this.domSanitizer.sanitize(SecurityContext.URL, url)
Update
For RC.6^ version use DomSanitizer
Plunker
And a good option is using pure pipe for that:
import { Pipe, PipeTransform } from '@angular/core';
import { DomSanitizer} from '@angular/platform-browser';
@Pipe({ name: 'safe' })
export class SafePipe implements PipeTransform {
  constructor(private domSanitizer: DomSanitizer) {}
  transform(url) {
    return this.domSanitizer.bypassSecurityTrustResourceUrl(url);
  }
} 
remember to add your new SafePipe to the declarations array of the AppModule. (as seen on documentation)
@NgModule({
   declarations : [
     ...
     SafePipe
   ],
})
html
<iframe width="100%" height="300" [src]="url | safe"></iframe>
Plunker
If you use embed tag this might be interesting for you:
Old version RC.5
You can leverage DomSanitizationService like this:
export class YourComponent {
  url: SafeResourceUrl;
  constructor(domSanitizationService: DomSanitizationService) {
    this.url = domSanitizer.bypassSecurityTrustResourceUrl('your url');
  }
}
And then bind to url in your template:
<iframe width="100%" height="300" [src]="url"></iframe>
Don't forget to add the following imports:
import { SafeResourceUrl, DomSanitizationService } from '@angular/platform-browser';
Plunker sample